1. Field of the Invention
The present invention relates generally to electron beam imaging apparatus and methods of using same.
2. Description of the Background Art
Automated inspection and review systems are important in process control and yield management for the semiconductor and related microelectronics industries. Such systems include optical and electron beam (e-beam) based systems.
In the manufacture of semiconductor devices, detection of physical defects and electrical failure earlier in the fabrication process is becoming increasingly important to shorten product development cycles and increase product yield and productivity. Advanced wafer inspection systems based on scanning electron microscopy technology have been used to detect defects and electrical failure as voltage contrast defects. However, as device design rules further shrink, and new processes (such as, for example, high aspect ratio (HAR) contacts in front-end-of-line (FEOL), HAR vias in back-end-of-line (BEOL), and dual damascene copper processes) are being widely implemented, it becomes more challenging to detect defects in device structures with smaller design rules and higher aspect ratios. Further, image contrast variation caused by uneven charge distribution can make e-beam inspection unstable or un-inspectable. Such contrast variation can occur from inside a die, from die to die, row to row, or wafer to wafer. In order to successfully inspect a wafer, control of surface charge is advantageous to 1) detect defects effectively, and 2) reduce image contrast variation during inspection.
In a conventional scanning electron microscope, a beam of electrons is scanned over a sample (for example, a semiconductor wafer). Multiple raster scans are typically performed over an area of the sample. The beam of electrons either interact with the sample and cause an emission of secondary electrons, bounce off the sample as backscattered electrons, or are absorbed by the sample. The secondary electrons and/or backscattered electrons are then detected by a detector that is coupled with a computer system. The computer system generates an image that is stored and/or displayed on the computer system.
Typically a certain amount of charge is required to provide a satisfactory image. This quantity of charge helps enhance the contrast features of the sample. Although conventional electron microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the sample for some applications.
For example, on a sample made of a substantially insulative material (e.g., silicon dioxide), performing one or more scans over a small area causes the sample to accumulate excess positive or negative charge in the small area relative to the rest of the sample. The excess of positive charge generates a potential barrier for some of the secondary electrons, and this potential barrier inhibits some of the secondary electrons from reaching the detector. Since this excess positive charge is likely to cause a significantly smaller amount of secondary electrons to reach the detector, an image of the small area is likely to appear dark, thus obscuring image features within that small area. Alternatively, excess negative charge build up on the sample can increase the collection of secondary electrons causing the image to saturate. In some cases, a small amount of charging is desirable since it can enhance certain image features (by way of voltage contrast) as long as it does not cause image saturation.
The excess charge remaining from a previous viewing or processing may therefore cause distortion. One solution used in SEM devices is to flood the sample with charged particles from a separate flood gun at a time separate from the inspection. The flooding beam is typically separate from the main inspection column of the e-beam inspection system, though in some instances the flooding beam may be implemented in the main inspection column. The intent of using such flooding is to equalize the charge appearing across the sample, thus improving contrast uniformity of the images. However, while flooding may equalize longer time scale charging effects, shorter time scale charge decay between the flooding and the imaging times may still cause undesirable charging distortion.
Some conventional systems average together multiple scans of a wafer surface to create an image for inspection. However, such averaging merely uses statistics to “hide” the undesirable effects of charging in the image.
FIG. 1 is a schematic diagram of a conventional electron beam imaging apparatus with a separate flood gun for charge control. As shown, the apparatus includes a primary (imaging) column 102. The column 102 includes an electron gun 104 that is the source of electrons for the primary (imaging) electron beam 106. Condensing lenses 108 condense the beam 106 into a tighter cross-section (and higher density). The beam 106 is controllably deflected using scanning coils 110 so as to scan the beam across the desired area. An objective lens 112 focuses the beam onto the wafer 114. A detector 116 is arranged to detect secondary electrons (and/or backscattered electrons). Electronics connected to the detector 116 is used to store the detected data so as to be able to form useful images for processing and analysis.
As further shown in FIG. 1, an electron beam flood gun 118 may be arranged so as to emit a flooding electron beam 120 over the sample 114. The use of this flood beam 120 to control charging is discussed below in relation to FIG. 2.
FIG. 2 is a flow chart of a conventional method of pre-flooding with a separate e-beam 120 and subsequently imaging the wafer 114 with a primary e-beam 106. In this conventional technique, before imaging, the wafer 114 is flooded 202 with electrons using the separate flooding e-beam 120. This flooding 202 is applied for a period of time sufficient to control a charge level at the wafer 114. Thereafter, the electron flooding 202 of the wafer 114 is stopped 204, and the imaging of the wafer 114 is subsequently begun. The primary e-beam 106 is scanned 206 across the wafer 114, and scattered (secondary and/or backscattered) electrons are detected 208. An image is formed 210 from the detected data. In some cases, several scans 206 may be used and the data may be averaged.
It is desirable to improve e-beam inspection and review apparatus. In particular, it is desirable to better control effects of charging and discharging on images used for inspection and review.